Turbochargers are well-known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housing.
In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted; an annular inlet passageway defined between facing radial walls arranged around the turbine chamber; an inlet arranged around the inlet passageway; and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel.
Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suite varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the annular inlet passageway.
Another known approach to improving turbocharging efficiency for an engine with a wide speed/load range is to provide a sequential two-stage turbocharging system, comprising one relatively small high-pressure turbocharger and another relatively large low-pressure turbocharger. The turbochargers are arranged in series so that exhaust from the engine flows first through the smaller turbine of the high-pressure turbocharger and then through the larger turbine of the low-pressure turbocharger. A valve-controlled bypass passage is provided for allowing exhaust gas to bypass the high-pressure turbine at high engine speeds and/or loads. Similarly, the compressors of the two turbochargers are also arranged in series, with air flowing first through the relatively large compressor of the low-pressure turbocharger and then through the relatively small compressor of the high-pressure turbocharger. Again, a valve-controlled bypass is provided to allow the inlet air to bypass the compressor of the high-pressure turbocharger at high engine speeds and/or loads.
Oxides of nitrogen (NOx), which are recognised to be harmful to the environment, are produced during the combustion process in an engine. In order to meet legislation intended to limit emissions exhaust gas recirculation (EGR) systems are used, in which a portion of the engine exhaust gas is recirculated through the combustion chambers. This is typically achieved by directing an amount of the exhaust gas from the exhaust manifold to the inlet manifold of the engine. The recirculated exhaust gas partially quenches the combustion process of the engine and hence lowers the peak temperature produced during combustion. As NOx production increases with increased peak temperature, recirculation of exhaust gas reduces the amount of undesirable NOx formed. Turbochargers may form part of the EGR system.
In order to introduce exhaust gas into the intake manifold, the recirculated exhaust gas must be at a higher pressure than that of the intake gas. However, in a turbocharged engine, the intake gas is typically at a pressure higher than that of the exhaust gas. This is due to the fact that the turbocharger compressor increases the pressure of the intake gas. As such, the pressure differential between the exhaust gas and intake gas is often in the incorrect direction to have flow from the exhaust system to the intake system.
A known EGR system for an engine with a turbocharger comprises a second turbocharger which operates in parallel with the standard turbocharger. The second turbocharger, herein known as the EGR turbocharger, has a turbine, which like the standard turbocharger, is powered by a portion of the engine exhaust; and a compressor which is fed with a portion of the engine exhaust gas, the compressor pressurising the exhaust gas and feeding it to the inlet manifold. As such, the turbine of the EGR turbocharger drives the EGR turbocharger compressor so that the EGR turbocharger acts as a pump, pumping a portion of engine exhaust gas to the engine intake. The EGR turbocharger turbine outlet is common with the turbine outlet of the standard turbocharger. As such, the EGR turbocharger is powered by the full pressure difference between the exhaust manifold of the engine and the (substantially) atmospheric pressure downstream of the standard turbocharger.
One difficulty with the use of an EGR turbocharger is that its efficiency is reduced owing to disparate pressure differences across the EGR turbocharger compressor and turbine respectively. In general the pressure difference across the EGR turbocharger turbine is much greater than that across the compressor. This is due to the fact that, in general, only a relatively low compression of the EGR gas by the EGR turbocharger compressor is required.